Recently the ATM and wireless technologies have gained a high visibility as they provide efficient and economical answers to requirements raised by new telecommunication services and by new end-user behaviors.
ATM has been recognized as the common base on which different types of services an ATM network can provide. The ATM technology can efficiently combine the transmission of speech, video, hi-fi sound (what is commonly called the multimedia traffic) and computer data into the wired network. Furthermore ATM has proven to scale well from very high speed network infrastructure (the information highways) to customer premises networks. One of the great advantages of the ATM technology is the fact that it can guarantee some level of service when an ATM connection is set up. Such guarantees can correspond to transmission rate, transmission latency and information loss. Such guarantees can be achieved mainly because the ATM architecture assumes that the transmission media are almost error free.
Wireless technology is currently becoming more and more successful as it allows mobile end-users to remain connected to their network and applications while being freed from a wired attachment. Different approaches are currently proposed for wireless communications: they differentiate by the services and the coverage they provide. Wireless Wide Area Networks (WAN) like the so-called GSM provide nation-wide coverage to the end-user, with modem equivalent transmission speeds. Wireless Data Packet networks provide nation-wide coverage to the end-user, with the so-called X.25 like transmission services. Finally Wireless Local Area Networks (LAN) provide establishment wide coverage with conventional LAN (e.g. Ethernet) equivalent communication services.
The combination of Wireless transmission with ATM technology is a new trend whose objective is to take advantage of both technologies, and for example to allow a mobile user to access an ATM network infrastructure while taking full advantage of all the facilities available with wired access to the ATM network. Since the main characteristics of each technology are very different, even incompatible, such a combination raises technical problems, as described hereinafter.
ATM technology characteristics
The ATM technology, as currently defined by standardization bodies such as the ATM Forum, and as currently implemented by commercial products, is characterized by the following main aspects:
1) Very short data granularity: in ATM network, all the traffic is carried within so-called ATM cells corresponding to a 53 bytes long structure made of a header field (5 bytes) and a payload field (48 data-bytes). PA1 2) The ATM cells are used either for control and signaling traffic or for information bearer traffic. Full duplex (FDX) links: on each ATM port, it is possible to simultaneously receive and transmit information. PA1 3) Point-to-point links: the media on which ATM cells are transmitted can only connect a pair of ATM stations. Although some ATM concentrator products can give the appearance of multidrop link, the ATM links have a point-to-point topology. PA1 4) Almost error free transmission: the transmission technologies currently used in ATM networks (e.g. over fiber or cable) provide typical bit error rate (BER) below 10.sup.-9. This capability is the main reason for permitting error recovery mechanisms within the ATM end stations and not within the ATM network.
In addition to these aspects, the ATM architecture further introduces the concept of ATM Service Class traffic parameters including Quality of Service (QoS) parameters to allow specifying, at ATM connection set-up, a traffic contract between the ATM end-point and the ATM network. These concepts formalize the characteristics of the user traffic so that each element of the ATM network involved in the end-to-end ATM connection may on one side reserve the necessary resources to provide the required service, and on the other side behave accordingly to the traffic contract. These concepts are summarized in the following table.
TABLE 1 __________________________________________________________________________ ATM Layer Services Categories Attribute CBR rt-VBR nrt-VBR ABR UBR Parameter __________________________________________________________________________ CLR Specified Specified Specified Specified Unspecified QoS peak-to-peak CDV Specified Specified Unspecified Unspecified Unspecified QoS Mean CTD Unspecified Unspecified Specified Unspecified Unspecified QoS Max. CTD Specified Specified Unspecified Unspecified Unspecified QoS PCR and CDVT Specified Specified Specified Specified Specified Traffic SCR and BT N/A Specified Specified N/A N/A Traffic MCR N/A N/A N/A Specified N/A Traffic Flow Control No No No Yes No __________________________________________________________________________ CBR: Constant Bit Rate rtVBR: real time Variable Bit Rate nrtVBR: non real time Variable Bit Rate ABR: Available Bit Rate UBR: Unspecified Bit Rate CLR: Cell Loss Rate CDV: Cell Delay Variation CTD: Cell Transfer Delay PCR: Peak Cell Rate CDVT: Cell Delay Variation Tolerance SCR: Sustainable Cell Rate BT: Burst Tolerance MCR: Minimum Cell Rate N/A: Not applicable
Finally, the channel should at least provide a capacity of 20 to 25 Mbps, in order to meet the throughput requirements of expected ATM applications. IBM Publication GG24-43300-00 entitled `Asynchronous Transfer Mode, Technical Overview` presents a broad overview of the ATM concept and details the ATM technology characteristics.
Wireless technology characteristics
The wireless transmissions are characterized by the following main aspects:
1) Poor efficiency to transmit short packets: before transmission of any piece of data over a radio channel, the transmitter must first issue a so-called Physical header which typically correspond to a significant portion of the bandwidth. In order to decrease the associated overhead, the piece of data which is transmitted next must be sufficiently large. For instance with conventional WLAN products, a frame of 1500 bytes (or more) follows the physical header.
2) Half Duplex (HDX) links: the only way to allow FDX operation over a radio channel is first to duplicate the radio modem transceiver (one part dedicated to transmission, the other to reception), and second to split by some means the frequency band into two sub-bands, each of them being used for a one-way transmission. Such an approach is quite expensive and also raises some technical difficulties such as the saturation of the receiving antenna by the emitting one. As a result the only technically and economically viable approach is to rely on a single radio transceiver piece of hardware, and to use the radio channel in HDX mode.
3) Broadcast transmission: the nature of the electromagnetic waves transmission is such that any receiver within range of a transmitter beam can get enough energy to receive information sent by the transmitter. Thus the wireless channel topology must be seen as point-to-multipoint. The use of directional antennas may reduce the reception area and thus the population of potential receivers, but it prevents any mobility of wireless stations. This last limitation is unacceptable for a wireless ATM network aimed to provide mobility support to mobile end-users. As a result, the wireless radio channel must be seen as a multidrop link.
4) High error rate: wireless transmission channels are characterized by poor quality figures. Typically the bit error rates observed on such wireless channels may be as bad as 10.sup.-3 or 10.sup.-4 : they are by far higher than what is assumed by ATM networks.
The `Mobile Communications Handbook` by Jerry D. Gibson, CRC IEEE Press, 1996, ISBN 0-8493-8573-3, gives an extensive description of the wireless technology. From the aforementioned points, it clearly appears that the wireless and ATM technologies significantly differ on important transmission characteristics which are resumed hereafter:
1) The ATM cells are so short that they would induce poor transmission efficiency if they were sent individually over the radio channel, as it is performed over conventional ATM transmission media. Radio transmissions cannot economically provide Full Duplex transmissions which are commonly used on ATM networks.
2) The topology of radio transmissions are not equivalent to the ones found in ATM networks.
3) The bit error rates observed over radio channels are by far worse that the ones assumed by the ATM architecture.
Some known techniques intend to address the technical challenges previously introduced, and may fill the gap between the wireless and the ATM technologies.
A first solution based on high speed point-to-point radio links may meet the requirements of a Wireless ATM network in terms of channel capacity, but cannot provide a satisfying answer to other needs. The main limitation is the point-to-point topology preventing wireless end-user to be mobile. In addition, the scheduling policies commonly used on such radio links have not been built to meet the QoS requirements of ATM connections.
Others solutions based on the conventional WLAN techniques are to some extent in line with some requirements of wireless access to an ATM network, but cannot address all of them. As far as the channel capacity is concerned, some techniques, as the one developed by the ETSI RES 10 , committee (the so-called HIPERLAN standard) provide a throughput which may be sufficient for some ATM scenarios. Concerning the channel topology and usage (FDX vs HDX), the U.S. Pat. No.: 5,384,777 to Ahmadi et al provides some techniques for defining a Wireless ATM network, but this solution suffers from important limitations. The most important one is the fact that the used traffic scheduling policy gives a solution to the support of conventional LAN traffic, but cannot guarantee at all the QoS requirements imposed by any type of ATM traffic. In addition to this limitation, the design point of traditional wireless LAN products is optimized for conventional LAN traffic based on large packets: 1.5 Kbytes or more) and therefore presents very poor efficiency figures if packets are replaced by ATM cells.
Therefore, it appears that the conventional wireless LAN technology does not fulfil the requirements of a wireless access to an ATM network.
Accordingly, it would be desirable to be able to provide a method capable of supporting any type of ATM service class with associated QoS parameters in a communication system providing a wireless access to an ATM network.
Furthermore, it would be desirable to be able to provide a system coordinating and optimizing the wireless channel bandwidth which is shared by all Mobile Terminals within a cell in a fair, flexible and ATM compatible manner and to achieve a high throughput.